Add in DOM and Styles.

@addyosmani would you mind checking this PR please? It's an initial commit of a DOM and Styles markdown file along with its resources. Some notes:

* `devtools-animation.js` line `632` contains a hardcoded path to where animations should live.
* Font files have been added which are fairly small (`~5kb`) for the play/pause icons.
* CSS has all been inlined into the markdown file.
* `<script>` tag has been added at the bottom of the markdown file. As it stands, it's currently a blocking `~100kb` JavaScript file (should be about half the size when minified and gzipped).
* The markdown file requests resources stored in the folder `dom-and-styles-files`.
* I haven't touched the `_book.yaml` file.
* Markdown isn't fully supported with the DevSite preview so I'm not entirely sure what the formatting will look like.

Please let me know what changes I can make if any. Thanks for your help!